Re: CVE-2021-4204: Linux Kernel eBPF Improper Input Validation Vulnerability

[tr3e wang <tr3e.wang () gmail com>]

Hi all,

This post is the exploit overview of CVE-2021-4202.

We successfully exploited this vulnerability to obtain full root
privileges on default installations of Ubuntu 20.04.

*Exploit overview*

1. We create a lot of BPF ringbufs, and choose one of them as victim.
   The BPF_FUNC_ringbuf_reserve allow us to have a pointer A to the
   beginning of the victim ringbuf's data field.

2. We do a pointer subtraction to point back to the victim ringbuf's
   mask field and overwrite it to 0x80000fff through
BPF_FUNC_ringbuf_submit.
   This allows us to do a limited out-of-bounds read/write. If lucky,
   we can read/write all the fields of the ringbuf behind the victim.

3. With the full control over all fields of the ringbuf behind the
   victim, we can manipulate the ringbuf to achieve a restricted
   address read/write with side effects in the vmalloc space.

4. We spawn many child processes, and use restricted address read to
   find the address of task_struct and cred in the vmalloc space.
   After zeroing out the uid/gid/... , full root privileges obtained.

Full exploit code will be published on github in the near future.

Regards,
tr3e